Shear Stress Measurement Apparatus

ABSTRACT

A shear stress sensor for measuring the shear force of a fluid flowing along a wall. A floating member, flush with the wall, senses a shear force of the flowing fluid. The floating member is mounted by support means to a base element that is placed in the wall, so that the floating member is flush with the wall and a shear force, sensed by the floating member, is translated via the support means to a Fiber Bragg Grating. The force acting on the Fiber Bragg Grating changes the shape and the refractive index of the Fiber Bragg Grating, thereby changing the resonant frequency of the Fiber Bragg Grating and causing a shift in the spectrum of wavelengths of light that is introduced to the Fiber Bragg Grating. This shift in the spectrum of wavelengths is representative of the shear force of the flowing fluid.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.12/313,922 filed Nov. 26, 2008 which is a non-provisional applicationbased on and claiming the priority of Provisional Application 60/990,352filed Nov. 27, 2007.

FIELD OF THE INVENTION

The present invention relates, in general, to measuring wall shearstress (also known as “skin friction”) in fluid flows and, inparticular, to apparatus for measuring skin friction by determining themagnitude of a shift in an optical spectrum.

BACKGROUND OF THE INVENTION

The accurate measurement of wall shear stress remains a challenge inmany industrial applications as well as in scientific research. Preciseknowledge of shear stress can benefit many fields of human activity. Forexample, it can (a) reduce the cost of manufacturing and increasequality and throughput of certain products in the pharmaceutical, food,paint, and coating industries, and (b) improve the performance ofaircraft. The real-time measurement of the local wall shear stress isimportant whenever dynamic flow control is required. Despite the longhistory of wall shear force measurement attempts using variousapproaches, the state of the art is still insufficient to meet allneeds.

The ways of measuring shear stress fall into three categories: Indirect,Semi-Direct and Direct.

Most of the available sensors for measuring shear stress use indirectmeasurement techniques where the wall shear stress is inferred, througha set of assumptions, from another flow property, such as, for example,streamwise velocity or heat transfer rate, measured at or near the wall.These Indirect measurement methods include, for example:

-   -   hot-wire/film-based anemometry (U.S. Pat. No. 5,883,310)    -   laser-based near-wall flow velocity measurements (D.        Fourguette, D. Modarress, D. Wilson, M. Koochesfahani, M.        Gharib, “An Optical MEMS-based Shear Stress Sensor for High        Reynolds Number Applications,” AIAA-2003-742, 41st Aerospace        Sciences Meeting and Exhibit, Reno, Nev., Jan. 6-9, 2003)

To retrieve information about shear stress, the indirect methods formeasuring shear stress require precise modeling of the flow near thewall and knowledge of flow parameters such, as temperature andviscosity. For most applications, these models are not developedsufficiently well and the parameters are not well known. Laser-basedflow velocity methods also require the fluid to be sufficientlytransparent for the laser radiation, thus restricting the field ofapplications of these methods.

Another method, that can be classified as Semi-Direct and that has beenfrequently used in aerodynamic applications, is the surfaceoil-film/liquid-crystal interferometry (see, for example, U.S. Pat. No.5,438,879). This approach, however, does not provide dynamic measurementof the wall shear stress and the spatial resolution can be poor.Technically, this approach requires covering an extended part of thewall with a film and having optical access to the film that can bedifficult to implement in applications other than aerodynamic. Also, forhigh levels of shear stress the film may be susceptible to mechanicaldamage.

Direct wall shear measurement techniques are preferable because theymeasure a motion of a floating element, positioned flush within thewall, that is directly caused by the shear force (U.S. Pat. No.4,464,928). In these methods, the measurement of the floating elementdisplacement is measured that is accomplished by a number of techniques:

A. Electrical

-   -   Piezoresistive—In this approach, the shift of the floating        element causes deformation of the piezoresistive element that is        translated into electric signal (see, for example, J. Shajii,        K-Y. Ng, M. Schmidt, “A Microfabricated Floating Element Shear        Stress Sensor Using Wafer-Bonding Technology,” Journal of        Microelectromechanical Systems, V. 1, No. 2, 1992, pp. 89-94)    -   Capacitor-based—In this approach, a floating element is mounted        on one of the capacitor plates, so that the shift of the        floating element changes the capacitance and this change is        measured by electrical/electronic means (for example, M.        Schmidt, R. Howe, S. Senturia, J. Artitonidis, “Design and        Falibration of a Microfabricated Floating-Element Shear-Stress        Sensor,” IEEE Transactions on Electron Devices, v35, n6, 1988,        pp. 750-757). These miniature electrical shear stress sensors,        while showing satisfactory results in laboratory tests, to date        have found limited applications due to following drawbacks:    -   (1) Small dynamic range of shear stress measurement    -   (2) Susceptibility to electromagnetic interference    -   (3) Low sensitivity for piezoresistive MEMS sensors    -   (4) For capacitance-based sensors, it is intrinsically difficult        to separate shear stress from pressure (or from the force        directed normal to the floating element surface)

B. Optical

-   -   Optical position measurement—In this approach, the floating        element is illuminated from above and the shift is measured by        an array of photodiodes placed below the element (A.        Padmanabhan, M. Sheplak, K. S. Breuer and M. A. Schmidt,        “Micromachined Sensors for Static and Dynamic Shear-Stress        Measurements in Aerodynamic Flows,” TRANSDUCERS '97, 1097        international Conference on Solid-state Sensors and Actuators,        Chicago, Jun. 16-19, pp. 137-141, 1997).    -   The basic difficulty of this method is the requirement of flow        to be transparent for the illuminating laser radiation that        should be arranged externally. This method is similar in design        to the oil film sensing described above.    -   Optical resonance methods—These methods rely on the deflection        of an optical beam to convert any change in a mechanical        attribute of a structure (for example, a displacement of the        cantilever supporting the floating element) into the resonance        frequency shift. Most popular shear stress sensors of this type        are fiber-based Fabry-Perot interferometers (U.S. Pat. No.        6,426,796 B1). The optical resonance methods are immune to        electromagnetic radiation and can be realized in a size that is        not larger than MEMS shear stress sensors described above. The        fiber-based Fabry-Perot sensors, however, require a delicate        mechanical alignment of the resonator (for example, rotational        motion of the floating element may cause significant loss of the        resonant signal quality). Another problem is the need for the        Fabry-Perot resonator to be optically clean, a condition that is        difficult to sustain in many applications.

The most important drawback of all known direct shear stress measurementmethods is the requirement of a sizeable gap between the floatingelement and the wall, to give room for the floating element to shiftunder the shear force. This gap needs to be greater than at least 100micrometers for all the described methods, to measure up to two ordersof magnitude in shear stress (with the exception of the fiber-basedFabry-Perot interferometry). Most liquids penetrate holes larger thanapproximately 1 micrometer. Therefore, in all existing directmeasurement shear stress sensors, the liquid will make its way into theinternal elements of the sensor and will fill the gap. This may causethe inner elements of the sensor to malfunction and may impede themotion of the floating element. The problem can be solved by inserting amaterial between the floating element and the wall or by covering thegap from the side of the flow with a flexible material, however, thatdecreases sensitivity of the sensor and may be unsuitable for chemicallyactive flows.

The drawback of the direct method is overcome in the approach that iscommonly known as “whispering gallery modes” (WGM) optical measurementtechnology. Like the Fabry-Perot interferometry method, the WGMtechnology is based on observing changes in the spectrum of a resonatorthat is subjected to the external force. Instead of using an openresonator, as it is done in the Fabry-Perot interferometry, WGM employsdielectric micro-resonators (such as a glass sphere) with light capturedinside. A minute change in the size, shape or refraction index of themicro-resonator alters the spectrum of the micro-resonator thatmanifests itself as a shift in its resonant frequency, a change in themagnitude for a particular resonance or in emergence of additionalresonances in the spectrum. The micro-resonator spectrum can bemeasured, for example, by using a tunable laser and an optical detector.Usually, the shifts of the resonances are most practical to measure.Therefore, the discussion below is restricted to measuring the resonanceshifts. The other features of the WGM spectra could also be employed inthe method.

The optical resonances, or “whispering gallery modes” (WGM), areextremely narrow. Thus very small shifts of WGMs can be detected, whichmay be used for the precise measurements of the force causing the shifts(M. Kozhevnikov, T. Ioppolo, V. Stepaniuk, V. Sheverev and V. Otugen,“Optical Force Sensor Based on Whispering Gallery Mode Resonators,”AIAA-2006-649, 44th AIAA Aerospace Sciences Meeting and Exhibit, Reno,Nev., Jan. 9-12, 2006). It has been shown that a change of amicro-sphere diameter as low as 0.01 nm can be detected by observing WGMshift (Ilchenko, V. S. et. al., “Strain-tunable high-Q opticalmicrosphere resonator,’ Optics Communications, 1998. 145(1-6): p.86-90). That provides an opportunity for designing a floating elementshear-stress sensor with an extremely narrow gap between the floatingelement and sensor wall. For example, for a gap of 100 nm that is notpenetratable by any liquid, three orders of magnitude for the force canbe measured.

A design for a shear stress sensor based on optical micro-resonators wasproposed by Otugen & Sheverev (V. Otugen, V. Sheverev, U.S. patentapplication Ser. No. 11/926,793 (November 2007, see also M. Kozhevnikov,T. Ioppolo, V. Stepaniuk, V. Sheverev and V. Otugen, “Optical ForceSensor Based on Whispering Gallery Mode Resonators,” AIAA-2006-649, 44thAIAA Aerospace Sciences Meeting and Exhibit, Reno, Nev., Jan. 9-12,2006). In this design, the micro-resonator is placed between a movablefloating element and the wall of the sensor, so that the micro-resonatoreffectively serves as a floating element support. Such an arrangementleads to the increased sensitivity of the sensor to the force actingnormal to the surface of the floating element. This force may shift thefloating element in the direction normal to the flow and change theposition of the micro-resonator relative to the optical waveguide. Theefficiency of the coupling will be affected which may lead to the WGMresonance shift caused by a normal force rather than shear force and,thus, to a false reading of the shear stress measurement.

SUMMARY OF THE INVENTION

A shear stress sensor for measuring the shear force of a flowing fluid,constructed in accordance with the present invention, includes a baseelement adapted for placement in a wall along which a fluid flows and afloating member for sensing a shear force of the fluid flowing along thewall. This shear stress sensor also includes a Fiber Bragg Grating thatchanges in shape and refractive index in response to a force applied tothe Fiber Bragg Grating that changes the resonant frequency of the FiberBragg Grating, whereby the spectrum of wavelengths of light introducedto the Fiber Bragg Grating shifts. Also included in this shear stresssensor are support means; positioned between the floating member and theFiber Bragg Grating, for mounting the floating member to the baseelement to be flush with the wall and transmitting a force sensed by thefloating member to the Fiber Bragg Grating to change the resonantfrequency of the Fiber Bragg Grating. This shear stress sensor furtherincludes optical carrier means for introducing light having a spectrumof wavelengths to the Fiber Bragg Grating and providing an indication ofa shift in the spectrum of wavelengths of the light caused by a changein the resonant frequency of the Fiber Bragg Grating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a sectional side view of a first embodiment of a shear stresssensor constructed in accordance with the present invention.

FIG. 1B is a sectional front view of the first embodiment of a shearstress sensor constructed in accordance with the present invention takenalong line B-B of FIG. 1A.

FIG. 2 is a sectional side view of a second embodiment of a shear stresssensor constructed in accordance with the present invention.

FIG. 3 is a sectional side view of a third embodiment of a shear stresssensor constructed in accordance with the present invention.

FIG. 4 is a sectional side view of a fourth embodiment of a shear stresssensor constructed in accordance with the present invention.

FIG. 5 is a sectional side view of a fifth embodiment of a shear stresssensor constructed in accordance with the present invention.

FIG. 6 is a schematic diagram of a system for measuring shear stressbased on a shift of optical resonances of a micro-resonator or a FiberBragg Grating constructed in accordance with the present invention.

FIG. 7 is a schematic diagram of the components of a first opticalcircuit for measuring shear stress based on a shift of opticalresonances of a micro-resonator or a Fiber Bragg Grating constructed inaccordance with the present invention.

FIG. 8 illustrates a shift of optical resonances of a micro-resonator asa force is applied to the micro-resonator.

FIG. 9 is a graph that presents an exemplary measurement of the forcevs. shift of the optical resonances of a micro-resonator.

FIG. 10A is a sectional side view of a sixth embodiment of a shearstress sensor constructed in accordance with the present invention.

FIG. 10B is a sectional front view of the sixth embodiment of a shearstress sensor constructed in accordance with the present invention takenalong line B-B of FIG. 10A.

FIG. 11A is a sectional side view of a seventh embodiment of a shearstress sensor constructed in accordance with the present invention.

FIG. 11B is a sectional front view of the seventh embedment of a shearstress sensor constructed in accordance with the present invention takenalong line B-B of FIG. 11A.

FIG. 12 is a side sectional view of an eighth embodiment of a shearstress sensor constructed in accordance with the present invention.

FIG. 13 is a side sectional view of a ninth embodiment of a shear stresssensor constructed in accordance with the present invention.

FIG. 14 is a schematic diagram of the components of a second opticalcircuit for measuring shear stress based on a shift of opticalresonances of two Fiber Bragg Gratings constructed in accordance withthe present invention.

FIG. 15 illustrates a shift of the optical spectrum of a Fiber BraggGrating as a force is applied to the Fiber Bragg Grating.

FIG. 16 is a graph that presents an exemplary measurement of the forcevs. shift of the optical spectrum of a Fiber Bragg Grating.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1A and 1B, a shear stress sensor 100, constructed inaccordance with the present invention, includes a base element 101, afloating member 102, support means in the form of a lever member 103, amicro-resonator 104, and optical carrier means in the form of an inputoptical carrier 105 and an output optical carrier 106. Sensor 100 may besecured to a wall 107 by mechanical or chemical means 108. Surface 109of floating element 102 is mounted flush with the test section of wall107 and the floating element can move a short distance (typically 0.1micrometers) along the flow direction represented by arrows 110. Levermember 103 is firmly attached at a first upper end 111 to floatingmember 102 and at a second lower end 112 to base element 101 in a mannerthat prevents vertical motion of the floating member. Unwanted verticalmovement of floating member 102 is excluded because lever member 103serves as a support member for the floating member and transfers shearforce to micro-resonator 104.

A shear force, applied to floating member 102, is amplified by levermember 103 as the lever member squeezes micro-resonator 104, which islocated between lever member 103 and base element 101. The amplifiedforce on micro-resonator 104 increases minimal detectable shear stress,effectively increasing the sensitivity of the sensor. The closermicro-resonator 104 is to the lower end 112 of lever member 103, thehigher the force amplification, and, therefore, the higher sensitivity.Depending on the application, the distance between micro-resonator 104and the lower end 112 of lever member 103 may be adjusted based on thesensitivity and measurement dynamic range requirements of theapplication. The inside wall of base element 101 can be formed with anindentation 114 to hold micro-resonator 104 in place and prevent itsmovement.

Light from a tunable narrowband source, such as a laser, passes throughinput optical carrier 105 and is coupled into micro-resonator 104. Lightfrom micro-resonator 104 is collected by output optical carrier 106 andtransmitted to an optical detector. A spectrum containing WGMs isrecorded by ramping the wavelength of the laser and recording thetemporal dependence of the detector output. The change in the shape andthe refractive index of micro-resonator 104, caused by being squeezedbetween lever member 103 and base element 101, alters the resonantfrequency of the micro resonator that is observed in the spectrum as ashift in WGMs. This shift is related to the shear stress using the knowntheoretical model or a calibration curve.

Floating member 102 may be made of the same or different material as thetest section wall 107. Surface 109 of floating member 102 may bemodified, for example by being polished or roughed, covered with othermaterial or be bare. The shape of the surface may repeat the shape ofthe surrounding wall and be plane, convex, concave or another.

The size of gaps 113 between floating member 102 and the inside walls ofbase element 101 can be less than one micrometer because the maximumdeformation of micro-resonator 104 can be of the order of a fewnanometers. Such small gaps will not disturb the flow and prevent fluidpropagation into the sensor, therefore, eliminating the need to covergaps 113 with flexible material.

Lever member 103 may have a rectangular or other cross-section. Thedimensions of lever member 103 may be optimized for the particularapplication. Lever member 103 can be made of different materials with adifferent modulus of elasticity to obtain an optimal response for thefluid flows with different expected shear stresses, while providing thenecessary support for floating element 102 to exclude unwanted verticalmovement of the floating member transverse to the flowing fluid thatpasses over the floating member.

Micro-resonator 104 may have various geometries, for example spherical,disk, elliptical and other and may be made of glass, plastic or othermaterial transparent to the interrogating light.

Although a single optical carrier (fiber or waveguide) can be used tocouple the interrogating light into micro-resonator 104 and to deliverthe transmitted light to an optical detector to provide an indication ofa shift in the spectrum of wavelengths of the light caused by a changein the resonant frequency of the micro-resonator, in many applicationsof the present invention employing separate optical carriers forintroducing light to the micro-resonator and collecting light from themicro-resonator will be preferred. This is shown in FIGS. 1A and 1B.Light is delivered to micro-resonator 104 by a first optical carrier 105and collected by a second optical carrier 106. In the FIGS. 1A and 1Bembodiment of the present invention, the input optical carrier 105 andthe output optical carrier 106 are coupled to micro-resonator 104 atdifferent places. Coupling may be facilitated by placing, in thevicinity of micro-resonator 104, a stretched fiber, an angle polishedfiber, a prism, or other means. An on-chip system with an etchedmicro-resonator and optical waveguides, for example, can be employed.The above elements can be manufactured using MEMs technology.

The general appearance of the WGM spectrum, when only one opticalcarrier is used, is typical of an absorption spectrum. The generalappearance of the WGM spectrum, when separate optical carriers are usedto introduce light to and collect light from the micro-resonator, istypical of an emission spectrum. Because the coupling efficiency isdifferent for different micro-resonator modes, the depths of the valleysor the heights of the peaks of the WGM spectrum also are different. Toaccurately find the position of a valley or a peak, the magnitude of thevalley or the peak should be larger than the noise level in thedetection system which is usually determined by the amplificationfactor. When one optical carrier is used, the amplification factor isfixed because it is defined by the light source intensity. When twooptical carriers are used, the amplification factor is determined by themagnitude of the highest peak of the WGM spectrum and can be 100-1000times higher, so that the signal-to-noise ratio also is higher. As aresult, weaker WGM resonances can be observed and their position can bedetermined more accurately which leads to improvement of the sensorsensitivity.

Another benefit of employing separate optical carriers for introducinglight to and collecting light from the micro-resonator is that, ingeneral, when a single optical carrier is used, the physical size of thesensor is likely to be larger than desired for the particularapplication. A single optical carrier configuration requires bending theoptical carrier inside the sensor housing to form a loop with a radiuslarge enough to avoid significant loss of light. When separate opticalcarriers are used, a return loop is not needed, so that the sensorphysical size is smaller.

Micro-resonator 104 may be placed between lever member 103 and theinside wall of base element 101, as shown in FIG. 1A, or between a levermember 201 and a micro-resonator holder member 202, as shown in the FIG.2, which illustrates a second embodiment of shear stress sensor 200constructed in accordance with the present invention. Components in FIG.2, corresponding to components in FIGS. 1A and 1B, have been given thesame reference numerals used in FIGS. 1A and 1B. Micro-resonator holdermember 202 can be formed with an indentation 203 to hold micro-resonator104 in place and prevent its movement. The position of micro-resonator104 may be adjusted to initially stress preload (compress) themicro-resonator by means of a preload component, such as a setscrew 204,that moves micro-resonator holder 202 away from or back to the wall ofbase element 101.

A third embodiment of a shear stress sensor 300, constructed inaccordance with the present invention, is illustrated in FIG. 3, inwhich components corresponding to components in FIGS. 1A and 1B havebeen given the same reference numerals used in FIGS. 1A and 1B. In thisthird embodiment, a lever member 301 is mounted for pivotal movement ina base element 302 to increase sensitivity of the sensor. Lever member301 can be mounted to base element 302 by means of a bearing 303.

Referring to FIG. 4, in which components corresponding to components inFIGS. 1A and 1B have been given the same reference numerals used inFIGS. 1A and 1B, a shear-stress sensor 400, constructed in accordancewith the present invention, has two micro-resonators 401 and 402. Onemicro-resonator is placed on each side of a lever member 403 thatsupports a floating member 404. Interrogating light is introduced tomicro-resonators 401 and 402 through input optical carriers 405 and 406,respectively, and is collected from the micro-resonators by two outputoptical carriers (not shown) individually associated with themicro-resonators and the input optical carriers in a manner similar tothe FIG. 1B embodiment of the present invention but which has only onemicro-resonator.

When shear force is applied to floating member 404, the shear force istransferred to micro-resonators 401 and 402 by lever member 403. Ifmicro-resonators 401 and 402 are preloaded, for example by preloadcomponents 407 and 408, respectively, application of the force to thetranslation member, namely lever member 403, leads to the squeezing ofone micro-resonator (e.g., micro-resonator 401) and the decompressing ofthe other micro-resonator (e.g., micro-resonator 402). WGM spectra fromthe two resonators will shift in opposite senses due to a change inshape and refractive index of micro-resonator 401 in a first sense and achange in shape and refractive index in a second and opposite sense ofmicro-resonator 402. Other factors, for example a change of the sensortemperature, may lead to the shift of the WGM spectra in the samedirection. Therefore, utilizing two micro-resonator sensors, asillustrated in FIG. 4, makes possible compensation of environmentalinfluence on the shear stress measurements.

FIG. 5 illustrates a fifth embodiment of a shear stress sensorconstructed in accordance with the present invention. The FIG. 5 shearstress sensor is similar to the shear stress sensor of FIGS. 1A and 1B,but adds an environmental sensor 501, for example a thermocouple or athermistor, that is positioned in proximity to micro-resonator 104 tosense temperature change. Environmental sensor 501 serves incompensating for spectrum shift in the micro-resonator due to theinfluence of an environmental factor, such as temperature. If thedependence of the spectrum shift versus change in the environmentalcondition, such as temperature, is known, for example from a theoreticalmodel or from calibration measurements, the measurement results can beadjusted according to the environmental sensor readings.

A second micro-resonator may serve as a reference for compensating forspectrum shift in micro-resonator 104 due to the influence of anenvironmental factor. This second micro-resonator would be installed inthe shear stress sensor in such a way that it is not acted on by acomponent, such as lever 103, that reacts to movement of floating member102, so that only environmental factors, such as sensor temperaturechange, will affect a spectrum shift of the second micro-resonator. Thespectrum shift of micro-resonator 104 can be recalculated taking intoaccount the spectrum shift of the second micro-resonator compensatingfor the environmental influence.

FIG. 6 illustrates a system for shear stress measurement constructed inaccordance with the present invention. This system includes a narrowbandtunable light source 601, an input optical carrier 602, an outputoptical carrier 603, a shear stress sensor 604, a detector 605, a signalconditioner 606, a data recorder 607, and a controller 608.

Light from narrowband tunable source 601, for example a tunable diodelaser, is directed through input optical carrier 602, for example anoptical fiber, to a shear stress sensor 604 in accordance with thepresent invention. The light wavelength is ramped between preset limits.Inside shear stress sensor 604, light is coupled into one or moremicro-resonator(s). Shear stress sensor 604 is installed flush with atest wall in such a way that a surface of the floating element of theshear stress sensor is exposed to the fluid flow and the shear forcethat is acting on it. Shear force, transferred to the micro-resonator(s)through the lever member, changes the shape and the index of refractionof the micro-resonator(s) which leads to a shift of the WGM resonances.Light emanating from the micro-resonator(s) is collected by an outputoptical carrier 603 and transmitted to detector 605, for example aphotodiode, that produces an electrical signal, the strength of which isrelated to the light intensity. The electrical signal is conditioned insignal conditioner 606, for example a photodiode amplifier, and recordedand stored by recorder 607, for example a computer with a dataacquisition card. The WGM spectrum is recorded when the light wavelengthis swept from the preset minimum to the preset maximum. Operation oflight source 601, detector 605, signal conditioner 606 and recorder 607is synchronized and controlled by a controller 608, for example acomputer with appropriate software. The shift of the resonances in theWGM spectrum can be determined by comparing the obtained WGM spectrumwith a WGM spectrum recorded before the force was applied to the sensor.Resonance shift then can be related to the shear stress using acalibration curve or theoretical formula.

A first exemplary optical circuit for shear stress measurement, inaccordance with the present invention, is shown in FIG. 7. Light from adiode laser 701 is transmitted through an optical fiber 702 to a wallshear stress sensor 703 and coupled to a micro-resonator 704. Lightemanating from micro-resonator 704 is directed to a photodiode 705through an optical fiber 706.

Two examples of measured WGM spectra are presented in FIG. 8. Spectrum801 represents no shear force being applied to the floating member ofthe sensor and spectrum 802 represents shear force being applied to thefloating member of the sensor. Shear force transferred by the levermember to the micro-resonator changes the shape and index of refractionof the micro-resonator, which leads to the WGM spectrum shift 803. Shearstress is determined by measuring the magnitude of the spectrum shiftand using a sensor calibration curve. An example of a calibration curveis given in FIG. 9.

The sixth embodiment of a shear stress sensor, constructed in accordancewith the present invention, that is illustrated in FIGS. 10A and 10B hasmuch in common with the embodiments of the present invention describedabove. Therefore, a detailed description of this sixth embodiment is notprovided. A Fiber Bragg Grating 900 serves as the component that changesin shape and refractive index and, therefore, resonant frequency,whereby the spectrum of wavelengths of light introduced to the FiberBragg Grating shifts. Fiber Bragg Grating 900 may be made of glass,plastic or other material transparent to interrogating light. The os1100Fiber Bragg Grating sold by Micron Optics can serve as this component.

A shear force, applied to floating member 102, is transferred to FiberBragg Grating 900 by lever member 103 to which the Fiber Bragg Gratingis affixed by suitable means, such as an epoxy. As lever member 103bends, Fiber Bragg Grating 900, also bends.

Interrogating light from a tunable narrowband source, such as a laser,passes through an optical carrier 901, such a fiber, and is coupled intoFiber Bragg Grating 900 and light from the Fiber Bragg Grating iscollected by optical carrier 901 and transmitted to an optical detectorto provide an indication of a shift in the spectrum of wavelengths ofthe light caused by a change in the resonant frequency of the FiberBragg Grating. This spectrum of wavelengths is recorded by ramping thewavelength of the laser and recording the temporal dependence of thedetector output. The change in the shape and the refractive index ofFiber Bragg Grating 900, caused by being bent by lever member 103,alters the resonant frequency of the Fiber Bragg Grating that isobserved as a shift of the spectrum. This shift is related to the shearstress using the known theoretical model or a calibration curve.

The general appearance of the Fiber Bragg Grating spectrum, when onlyone optical carrier serves to both introduce light to the Fiber BraggGrating and collect light from the Fiber Bragg Grating, is typical of areflection spectrum. The general appearance of the Fiber Bragg Gratingspectrum, when, as described below, one optical carrier serves tointroduce light to the Fiber Bragg Grating and another optical carrierserves to collect light from the Fiber Bragg Grating, is typical of atransmission spectrum. A benefit of employing a single optical carrier901 for introducing light to and collecting light from Fiber BraggGrating 900 is that, in general, when two optical carriers are used, thephysical size of the sensor might be larger than desired for aparticular application.

A shear stress sensor, constructed in accordance with the presentinvention, that is illustrated in FIGS. 11A and 11B differs from thesixth embodiment described above in that light from a tunable narrowbandsource, such as a laser, passes through an input optical carrier 902 andis coupled into a Fiber Bragg Grating 903 and light from the Fiber BraggGrating is collected by an output optical carrier 904 and transmitted toan optical detector. As shown in FIG. 11B, output optical carrier 904 isbent to form a loop 905 that has a radius large enough to avoidsignificant light loss. Otherwise, the sixth and seventh embodiments areidentical.

The eighth embodiment of a shear stress sensor, constructed inaccordance with the present invention, that is illustrated in FIG. 12includes two Fiber Bragg Gratings 906 and 907, disposed on oppositesides of lever member 103, into which interrogating light is introducedand from which light is collected by optical carriers 908 and 909,respectively. When a shear force is applied to floating member 102, theshear force is transferred to Fiber Bragg Gratings 906 and 907 by levermember 103. Application of this force to lever member 103 leads tostraining of Fiber Bragg Grating 906 and stressing of Fiber BraggGrating 907. Spectra from the two Fiber Bragg Gratings shift in oppositesenses due to the change in shape and refractive index of Fiber BraggGrating 906 in one sense and the change in shape and refractive index ofFiber Bragg Grating 907 in a second and opposite sense. Other factors,for example, a change in sensor temperature, might lead to the shift ofthe spectra in the same direction. Therefore, utilizing two Fiber BraggGratings, as illustrated in FIG. 12, makes possible compensation ofenvironmental influence on the shear stress measurements. It will beapparent that the FIG. 12 embodiment of the present invention can bearranged with separate input optical carriers and output opticalcarriers for introducing light to and collecting light from Fiber BraggGratings 906 and 907.

FIG. 13 illustrates a ninth embodiment of a shear stress sensorconstructed in accordance with the present invention. The FIG. 13 shearstress sensor is similar to the shear stress sensor of FIGS. 10A and10B, but adds an environmental sensor 910, for example a thermocouple ora thermistor, that is positioned in proximity to Fiber Bragg Grating 900to sense temperature change. Environmental sensor 910 serves incompensating for spectrum shift in Fiber Bragg Grating 910 due to theinfluence of an environmental factor, such as temperature. If thedependence of the spectrum shift versus change in the environmentalcondition, such as temperature, is known, for example from a theoreticalmodel or from calibration measurements, the measurement results can beadjusted according to the environmental sensor readings.

A second Fiber Bragg Grating may serve as a reference for compensatingfor spectrum shift in Fiber Bragg Grating 910 due to the influence of anenvironmental factor. This second Fiber Bragg Grating would be installedin the shear stress sensor in such a way that it is not acted on by acomponent, such as lever 103, that reacts to movement of floating member102, so that only environmental factors, such as sensor temperaturechange, will affect a spectrum shift of the second Fiber Bragg Grating.The spectrum shift of Fiber Bragg Grating 900 can be recalculated takinginto account the spectrum shift of the second Fiber Bragg Gratingcompensating for the environmental influence.

When a Fiber Bragg Grating is substituted for a micro-resonator in thefirst exemplary optical circuit illustrated in FIG. 7, the opticalcircuit operates in the same manner as described above.

A second exemplary optical circuit for shear stress measurement, inaccordance with the present invention, is shown in FIG. 14. The FIG. 14optical circuit differs from the FIG. 7 optical circuit in that lightsource 911 is a broadband light source, identified as a light emittingdiode, and the shear stress sensor 703 includes a pair of Fiber BraggGratings 912 and 913 instead of a micro-resonator. Light is transmittedfrom light emitting diode 911 through an optical fiber 702 to shearstress sensor 703 and coupled to Fiber Bragg Grating 912. The spectrumof light emanating from Fiber Bragg Grating 912 is a transmissionspectrum when the Fiber Bragg Grating is installed in the sensor asshown in FIGS. 11A and 11B or a reflection spectrum if the Fiber BraggGrating is installed in the sensor as shown in FIGS. 10A and 10B. Lightemanating from Fiber Bragg Grating 912 is then directed to Fiber BraggGrating 913 via an optical fiber 914. Fiber Bragg Grating 913 may beinstalled to transmit when arranged as illustrated in FIGS. 11A and 11Bor reflect when arranged as illustrated in FIGS. 10A and 10B. Lightemanating from Fiber Bragg Grating 913 will carry information aboutspectra of both of the Fiber Bragg Gratings. This light is directedthrough an optical fiber 706 to a photodiode 707 that produces anelectrical signal, the strength of which is related to light intensity.When shear force is applied to the floating element of the sensor, theresonance frequencies of Fiber Bragg Gratings 912 and 913 shift inopposite senses changing the spectrum of light emanating from the FiberBragg Grating 913 which results in a change in the output of photodiode707. This change in the output of photodiode 707 then can be related tothe shear stress using a calibration curve or a theoretical formula.

Two examples of measured Fiber Bragg Grating spectra are presented inFIG. 15. Spectrum 920 represents no shear force being applied to thefloating member of the sensor and spectrum 921 represents shear forcebeing applied to the floating member of the sensor. Shear forcetransferred by the lever member to the Fiber Bragg Grating changes theshape and index of refraction of the Fiber Bragg Grating, which leads tothe Fiber Bragg Grating spectrum shift 922. Shear stress is determinedby measuring the magnitude of the spectrum shift and using a sensorcalibration curve. An example of a calibration curve is given in FIG.16.

Although the invention is illustrated and described herein withreference to specific embodiments; the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

1. A shear stress sensor for measuring the shear force of a flowingfluid comprising: a base element adapted for placement in a wall alongwhich a fluid flows; a floating member for sensing a shear force of thefluid flowing along the wall; a Fiber Bragg Grating that changes inshape and refractive index in response to a force applied to said FiberBragg Grating that changes the resonant frequency of said Fiber BraggGrating, whereby the spectrum of wavelengths of light introduced to saidFiber Bragg Grating shifts; support means, positioned between saidfloating member and said Fiber Bragg Grating, for: (a) mounting saidfloating member to said base element to be flush with the wall, and (b)transmitting a force sensed by said floating member to said Fiber BraggGrating to change the resonant frequency of said Fiber Bragg Grating;and optical carrier means for: (a) introducing light having a spectrumof wavelengths to said Fiber Bragg Grating, and (b) providing anindication of a shift in the spectrum of wavelengths of the light causedby a change in the resonant frequency of said Fiber Bragg Grating.
 2. Ashear stress sensor according to claim 1 wherein said support means is alever having a first end attached to said floating member and a secondend attached to said base element.
 3. A shear stress sensor according toclaim 1 further including means for sensing an environmental factor inthe proximity of said Fiber Bragg Grating to compensate for spectrumshift in said Fiber Bragg Grating due to the influence of theenvironmental factor.
 4. A shear stress sensor according to claim 2wherein said optical carrier means include: (a) a first optical carrierfor introducing to said Fiber Bragg Grating light having a firstspectrum of wavelengths, and (b) a second optical carrier for collectingfrom said Fiber Bragg Grating light having a second spectrum ofwavelengths.
 5. A system for measuring shear stress comprising: a shearstress sensor including: (a) a base element adapted for placement in awall along which a fluid flows, (b) a floating member for sensing ashear force of the fluid flowing along the wall, (c) a Fiber BraggGrating that changes in shape and refractive index in response to aforce applied to said Fiber Bragg Grating that changes the resonantfrequency of said Fiber Bragg Grating, whereby the spectrum ofwavelengths of light introduced to said Fiber Bragg Grating shifts, (d)support means, positioned between said floating member and said FiberBragg Grating, for: (1) mounting said floating member to said baseelement to be flush with the wall, and (2) transmitting a force sensedby said floating member to said Fiber Bragg Grating to change theresonant frequency of said Fiber Bragg Grating, and (e) optical carriermeans for: (1) introducing light having a spectrum of wavelengths tosaid Fiber Bragg Grating, and (2) providing an indication of a shift inthe spectrum of wavelengths of the light caused by a change in theresonant frequency of said Fiber Bragg Grating; a light source forsupplying light to said optical carrier means; a detector for receivinglight from said optical carrier means for producing an electrical signalrepresentative of the intensity of the light received from said opticalcarrier means; and means for recording the intensity of the lightreceived from said optical carrier means.
 6. A shear stress sensoraccording to claim 2 further including means for sensing anenvironmental factor in the proximity of said Fiber Bragg Grating tocompensate for spectrum shift in said Fiber Bragg Grating due to theinfluence of the environmental factor.
 7. A shear stress sensor formeasuring the shear force of a flowing fluid comprising: a base elementadapted for placement in a wall along which a fluid flows; a floatingmember for sensing a shear force of the fluid flowing along the wall; afirst Fiber Bragg Grating that changes in shape and refractive index inresponse to a force applied to said first Fiber Bragg Grating thatchanges the resonant frequency of said Fiber Bragg Grating, whereby thespectrum of wavelengths of light introduced to said first Fiber BraggGrating shifts; a second Fiber Bragg Grating that changes in shape andrefractive index in response to a force applied to said second FiberBragg Grating that changes the resonant frequency of said second FiberBragg Grating, whereby the spectrum of wavelengths of light introducedto said second Fiber Bragg Grating shifts; support means, positioned:(a) between said floating member and said first Fiber Bragg Grating, and(b) between said floating member and said second Fiber Bragg Gratingfor: (a) mounting said floating member to said base element to be flushwith the wall, and (b) transmitting a force by said floating member to:(1) said first Fiber Bragg Grating to change the resonant frequency ofsaid first Fiber Bragg Grating, and (2) said second Fiber Bragg Gratingto change the resonant frequency of said second Fiber Bragg Grating; andoptical carrier means for: (a) introducing light having a spectrum ofwavelengths to said first Fiber Bragg Grating and to said second FiberBragg Grating, and (b) providing indications of shifts in the spectra ofwavelengths of the light caused by changes in the resonant frequenciesof said first Fiber Bragg Grating and said second Fiber Bragg Grating.8. A shear stress sensor according to claim 7 wherein said opticalcarrier means include: (a) a first optical carrier for introducing thelight having the first spectrum of wavelengths to said first Fiber BraggGrating and a second optical carrier for collecting the light from saidfirst Fiber Bragg Grating having the second spectrum of wavelengths, and(b) a third optical carrier for introducing the light having the firstspectrum of wavelengths to said second Fiber Bragg Grating and a fourthoptical carrier for collecting the light from said second Fiber BraggGrating having the second spectrum of wavelengths.
 9. A shear stresssensor according to claim 7 wherein said support means is a lever havinga first end attached to said floating member and a second end attachedto said base element.
 10. A shear stress sensor according to claim 8wherein said support means is a lever having a first end attached tosaid floating member and a second end attached to said base element. 11.A shear stress sensor for measuring the shear force of a flowing fluidcomprising: a base element adapted for placement in a wall along which afluid flows; a floating member for sensing a shear force of the fluidflowing along the wall; a first Fiber Bragg Grating that changes inshape and refractive index in response to a force applied to said firstFiber Bragg Grating that changes the resonant frequency of said FiberBragg Grating, whereby the spectrum of wavelengths of light introducedto said first Fiber Bragg Grating shifts; a second Fiber Bragg Gratingthat changes in shape and refractive index in response to a forceapplied to said second Fiber Bragg Grating that changes the resonantfrequency of said second Fiber Bragg Grating, whereby the spectrum ofwavelengths of light introduced to said second Fiber Bragg Gratingshifts; support means, positioned: (a) between said floating member andsaid first Fiber Bragg Grating, and (b) between said floating member andsaid second Fiber Bragg Grating for: (a) mounting said floating memberto said base element to be flush with the wall, and (b) transmitting aforce by said floating member to: (1) said first Fiber Bragg Grating tochange the resonant frequency of said first Fiber Bragg Grating, and (2)said second Fiber Bragg Grating to change the resonant frequency of saidsecond Fiber Bragg Grating; and optical carrier means for: (a)introducing light having a first spectrum of wavelengths to said firstFiber Bragg Grating, (b) collecting light of a second spectrum ofwavelength emanating from said first Fiber Bragg Grating and introducingthe light having the second spectrum of wavelengths to said second FiberBragg Grating, and (c) providing indications of shifts in the spectra ofwavelengths of the light caused by changes in the resonant frequency ofsaid first Fiber Bragg Grating and changes in the resonant frequency ofsaid second Fiber Bragg Grating.
 12. A shear stress sensor according toclaim 11 wherein said optical carrier means include: (a) a first opticalcarrier for introducing the light having the first spectrum ofwavelengths to said first Fiber Bragg Grating, (b) a second opticalcarrier for collecting the light from said first Fiber Bragg Gratinghaving the second spectrum of wavelengths and introducing the lighthaving the second spectrum of wavelengths to said second Fiber BraggGrating, and (b) a third optical carrier for collecting the light fromsaid second Fiber Bragg Grating having the second spectrum ofwavelengths.
 13. A shear stress sensor according to claim 11 whereinsaid support means is a lever having a first end attached to saidfloating member and a second end attached to said base element
 14. Ashear stress sensor according to claim 12 wherein said support means isa lever having a first end attached to said floating member and a secondend attached to said base element.
 15. A system for measuring shearstress according to claim 5 wherein said light source supplies lighthaving a ramped wavelength.
 16. A system for measuring shear stressaccording to claim 5 wherein said light source is a broadband lightsource.